This invention generally relates to variable data rate transmissions and, more particularly, to techniques for efficiently allocating spreading codes for variable rate data transmissions.
Cellular radio communication systems have recently been developed that use spread spectrum modulation and code division multiple access (CDMA) techniques. In a typical direct sequence CDMA system, an information data stream to be transmitted is superimposed on a much-higher-symbol-rate data stream sometimes known as a spreading sequence. Each symbol of the spreading sequence is commonly referred to as a chip. Each information signal is allocated a unique spreading code that is used to generate the spreading sequence typically by periodic repetition. The information signal and the spreading sequence are typically combined by multiplication in a process sometimes called coding or spreading the information signal. A plurality of spread information signals are transmitted as modulations of radio frequency carrier waves and are jointly received as a composite signal at a receiver. Each of the spread signals overlaps all of the other coded signals, as well as noise-related signals, in both frequency and time. By correlating the composite signal with one of the unique spreading sequences, the corresponding information signal can be isolated and decoded.
As radiocommunication becomes more widely accepted, it will be desirable to provide various types of radiocommunication services to meet consumer demand. For example, support for facsimile, e-mail, video, internet access, etc. via radiocommunication systems is envisioned. Moreover, it is expected that users may wish to access different types of services at the same time. For example, a video conference between two users would involve both speech and video support. Some of these different services will require relatively high data rates compared with speech service that has been conventionally supplied by radio communication systems, while other services will require variable data rate service. Thus, it is anticipated that future radio communication systems will need to be able to support high data rate communications as well as variable data rate communications.
Several techniques have been developed to implement variable rate communications in CDMA radio communication systems. From the perspective of transmitting data at varying rates, these techniques include, for example, discontinuous transmission (DTX), variable spreading factors, multi-code transmission and variable forward error correction (FEC) coding. For systems employing DTX, transmission occurs only during a variable portion of each frame, i.e., a time period defined for transmitting a certain size block of data. The ratio between the portion of the frame used for transmission and the total frame time is commonly referred to as the duty cycle .gamma.. For example, when transmitting at the highest possible rate, i.e., during the entire frame period, .delta.=1, while for zero rate transmissions, e.g., during a pause in speech, .delta.=0. DTX is used, for example, to provide variable data rate transmissions in systems designed in accordance with the U.S. standard entitled "Mobile Station-Base Station Compatibility Standard for Dual-Mode Wideband Spread Spectrum Cellular System", TIA/EIA Interim Standard TIA/EIA/IS-95 (July 1993) and its revision TIA/EIA Interim Standard TIA/EIA/IS-95-A (May 1995). Such standards that determine the features of U.S. cellular communication systems are promulgated by the Telecommunications Industry Association and the Electronic Industries Association located in Arlington, Va.
Varying the spreading factor is another known technique for providing variable data rate communication. As mentioned above, DS-CDMA spread spectrum systems spread data signals across the available bandwidth by multiplying each of the data signals with spreading sequences. By varying the number of chips per data symbol, i.e., the spreading factor, while keeping the chip rate fixed, the effective data rate can be controllably varied. In typical implementations of the variable spreading factor approach, the spreading factor is limited by the relationship to SF=2.sup.k =SF.sub.min where SF.sub.min is the minimum allowed spreading factor corresponding to the highest allowed user rate.
Another known technique for varying the transmitted data rate is commonly referred to as multi-code transmission. According to this technique, data is transmitted using a variable number of spreading codes where the exact number of codes used depends on the instantaneous user bit rate. Effectively, this means allocating a variable number of physical channels to a connection to provide variable bandwidth. An example of multi-code transmission is described in U.S. Pat. application Ser. No. 08/636,648 entitled "Multi-Code Compressed Mode DS-CDMA Systems and Methods", filed on Apr. 23, 1996, the disclosure of which is incorporated here by reference.
Yet another technique for varying the transmitted data rate in radio communication systems involves varying the FEC. More specifically, the rate of the forward error correction (FEC) coding is varied by using code-puncturing and repetition or by switching between codes of different rates. In this way the user rate is varied while the channel bit rate is kept constant. Those skilled in the art will appreciate the similarities between varying the FEC and a variable spreading factor as mechanisms to implement variable rate transmission.
In both the uplink and downlink, it is desirable that any number of logical channels can be transmitted simultaneously to support a single connection between a base station and a mobile station to support various data rates. To transmit these logical channels over the radio interface, a number of physical channels are allocated. These physical channels are separated using different spreading codes (channelization codes), i.e., multicode transmission is used. Each physical channel can have one of several possible data rates, i.e., one of several possible spreading factors is used when spreading the data transmitted on the physical channel. To date, however, a flexible solution which allocates code words to physical channels taking into consideration the codes which have already been allocated to other channels and power considerations associated with the in-phase (I) and quadrature (Q) transmitter branches has not been provided.
Accordingly, it would be desirable to create new techniques and systems for allocating spreading codes in a flexible manner that supports multicode transmissions and variable spreading factors, and that optimizes power efficiency.